Methods and apparatus for cleaning semiconductor substrates after polishing of copper film

ABSTRACT

A cleaning solution, method, and apparatus for cleaning semiconductor substrates after chemical mechanical polishing of copper films is described. The present invention includes a cleaning solution which combines deionized water, an organic compound, and a fluoride compound in an acidic pH environment for cleaning the surface of a semiconductor substrate after polishing a copper layer. Such methods of cleaning semiconductor substrates after copper CMP alleviate the problems associated with brush loading and surface and subsurface contamination.

This application is in divisional of Ser. No. 08/955,393 filed Oct. 21,1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for processing andcleaning a substrate, and more specifically to methods and apparatus forcleaning semiconductor substrates after polishing of copper films.

2. Background Information

In the manufacture of advanced semiconductor devices, copper (Cu) isbeginning to replace aluminum (A1) as the material for metallization. Cuhas become desirable due to its lower resistivity and significantlyimproved electromigration lifetime, when compared to A1.

One process for Cu metallization uses a dual damascene approach. Asillustrated in FIG. 1a, a dielectric layer 110 is deposited above asubstrate 100. Dielectric layer 120 may be made up of materials such assilicon dioxide. Vias and/or trenches 120 are then formed in thedielectric layer 110, as illustrated in FIG. 1b. Vias/trenches 120 maybe formed, for example, using dry etching techniques. Next, a thin layerof barrier material (barrier layer) 130, for example, tantalum (Ta),titanium (Ti), or titanium nitride (TiN) is deposited as illustrated inFIG. 1c. After barrier layer 130 is deposited the vias/trenches copper110 are filled with copper (Cu) layer 140, as illustrated in FIG. 1d.Such as copper layer 140 may be deposited using well known depositiontechniques, for example, chemical vapor deposition (CVD), physical vapordeposition (PVD), or electroplating. In order to isolate the copperinterconnects, as illustrated in FIG. 1e, the excess copper layer 140and barrier layer 130 must be removed.

One method for removing the excess copper layer 140 and barrier layer130 is polishing the surface of the substrate, for example, polishingusing chemical mechanical polishing (CMP). In a CMP process, thesemiconductor substrate is polished with a slurry containing abrasiveparticles, such as alumina particles, and an oxidant, such as hydrogenperoxide. In the CMP process, contaminants are introduced which includeparticles and/or metal contamination on the copper surface 150,dielectric surface 160, and in the dielectric subsurface 165.

Regardless of how the CMP process is performed, the surface ofsemiconductor substrate must be cleaned of contaminants. If not removed,these contaminants may affect device performance characteristics and maycause device failure to occur at faster rates than usual. Cleaning thesemiconductor substrate after chemical mechanical polishing of coppermay be necessary to remove such contaminants from the copper layer anddielectric layers.

One method for cleaning the semiconductor substrate after polishing ofthe copper layer is brush scrubbing. Brush scrubbing, whethersingle-sided or double-sided brush scrubbing, is the industry standardfor cleaning oxide and tungsten CMP applications. However, there areseveral problems associated with applying brush scrubbing to post copperCMP cleaning.

One such problem is brush loading. During the CMP process, the topsurface of the copper layer may be oxidized and forms copper oxide, forexample copper oxide (Cu₂ O or CuO) or copper hydroxide (Cu(OH)₂). Inbasic or neutral pH cleaning environments, the copper oxide or copperhydroxide does not dissolve and may be transferred to the brushes, thusloading the brushes. The contaminated (or loaded) brushes may thentransfer the copper oxide or copper hydroxide contaminants tosubsequently processed substrates during cleaning.

For tungsten and other oxide applications, brush loading could becurtailed by adding a dilute ammonium hydroxide (NH₄ OH). In thepresence of NH₄ OH, part of the copper oxide may form Cu(NH₃)²⁺ complexand may be dissolved; however, due to the high pH environment, thedilute ammonium hydroxide has been found to be insufficient to preventbrush loading of copper oxide. Additionally, it has been found thatscrubbing with dilute ammonium hydroxide also causes etching of thecopper layer and may cause serious surface roughening.

Brush loading may also occur when alumina particles are used in thecopper CMP process. In neutral or inorganic acid (e.g. HCl) cleaningenvironments, there is an electrostatic attraction between aluminaparticles and the silicon dioxide surface which makes it difficult toremove the alumina particles from the surface of the dielectricmaterial. Because of the electrostatic attractive force, the aluminaparticles may also adhere to the brush and cause another brush loadingproblem with similar effects to those discussed above.

Yet another problem caused by the CMP process is that the surface andsubsurface of the dielectric layer may become contaminated duringpolishing with metal from the copper layer and barrier layer as well asother contaminants from the slurry. During the CMP process,contaminants, especially metal contaminants, may penetrate into thedielectric layer up to approximately 100 angstroms (Å) from the surface.Again, these contaminants may affect device performance characteristicsand may cause device failure.

Thus, what is needed is a cleaning environment and methods for cleaningpost copper CMP substrates that alleviate the problems of brush loadingwithout affecting the quality of the copper and dielectric layers.Furthermore, what is needed is a cleaning environment and methods forcleaning post copper CMP substrates that have the capability of removingsurface and subsurface contaminants from the copper and dielectriclayers.

SUMMARY OF THE INVENTION

A cleaning solution, methods and apparatus for cleaning semiconductorsubstrates after chemical mechanical polishing of copper films isdescribed. The present invention includes a cleaning solution whichcombines deionized water, an organic compound, and a fluoride compoundin an acidic pH environment for cleaning the surface of a semiconductorsubstrate after polishing a copper layer.

Additional features and benefits of the present invention will becomeapparent from the detailed description, figures, and claims set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures in which:

FIG. 1a illustrates a semiconductor substrate having a dielectric layerdeposited thereon.

FIG. 1b illustrates the semiconductor substrate of FIG. 1a after viasand/or trenches are formed in the dielectric layer.

FIG. 1c illustrates the semiconductor substrate of FIG. 1b after a thinbarrier layer has been deposited thereon.

FIG. 1d illustrates the semiconductor substrate of FIG. 1c after a layerof Copper material has been deposited thereon.

FIG. 1e illustrates the semiconductor substrate of FIG. 1d afterchemical mechanical polishing of the excess copper layer and barrierlayer.

FIG. 2 illustrates one embodiment of a scrubber system.

FIG. 3 illustrates a flowchart of one embodiment of the process of thepresent invention.

DETAILED DESCRIPTION

Methods and Apparatus for Cleaning Semiconductor Substrates AfterPolishing of Copper Film are disclosed. In the following description,numerous specific details are set forth such as specific materials,processes, parameters, dimensions, etc. in order to provide a thoroughunderstanding of the present invention. It will be obvious, however, toone skilled in the art that these specific details need not be employedto practice the present invention. In other instances, well knownmaterials or methods have not been described in detail in order to avoidunnecessarily obscuring the present invention.

The following description describes a cleaning solution, methods, andapparatus for cleaning a semiconductor substrate after the formation ofcopper interconnect(s) and chemical mechanical polishing(CMP)/planarization of that copper interconnect(s). It should be notedthat the processes for formation of copper interconnects insemiconductor device fabrication are well known in the art and aretherefore not described in detail herein.

It should also be noted that, although the term semiconductor substrateis used throughout this disclosure and claims the term semiconductorsubstrate is used to refer to a silicon semiconductor substrate or apart thereof, such as gallium arsenide, upon which device layers havebeen or are going to be formed. It should also be noted that the termsubstrate includes but is not limited to: fully processed,semi-processed, or unprocessed substrates with semiconductor materialsthereon.

Additionally it should be noted that, although the present invention isdescribed in conjunction with the scrubbing of a semiconductor substrateor wafer, it will be appreciated that any similarly shaped, i.e.generally flat substrate, may be processed by the methods andapparatuses of the present invention. Further, it will be appreciatedthat reference to a semiconductor substrate or wafer may include a bareor pure semiconductor substrate, with or without doping, a semiconductorsubstrate with epitaxial layers, a semiconductor substrate incorporatingone or more device layers at any stage of processing, other types ofsubstrates incorporating one or more semiconductor layers such assubstrates having semiconductor on insulator (SIO) devices, orsubstrates for processing other apparatuses and devices such as flatpanel displays, multichip modules, etc.

As described in the background of the invention after the copperinterconnects on a semiconductor substrate have been planarized usingCMP techniques, it is necessary to clean the semiconductor substrate andremove any contaminants from the surface and subsurface of thesemiconductor substrate. One such technique for removing contaminantsfrom the semiconductor substrate is scrubbing the semiconductorsubstrate (substrate).

As an example, and not by limitation, the present invention is describedin conjunction with a scrubbing process, more specifically, a scrubbingprocess in which both sides of the wafer are scrubbed simultaneously.The scrubber may include a number of stations. Each of these stationsrepresents one or more steps in the substrate cleaning process.Contaminated substrates are loaded at one end of the system and cleanedand dried substrates are unloaded from the other end of the system.Example of a systems of this type are the DSS-200™ Scrubber and theSynergy™ Scrubber available from Lam Research Corporation (formerlyOnTrack Systems, Inc.) of Fremont, Calif.

FIG. 2 represents a cross sectional view of a Synergy configuration(cleaning system). Usually, the contaminated substrates are delivered tothe cleaning system after chemical mechanical planarization (CMP), froma wet bench, or from other processes resulting in contamination. At thestart of the cleaning process contaminated substrates are loaded into awafer cassette 280 (cassette) and the cassette 280 is then placed intothe wet send indexer station 210. After cassette 280 is placed into wetsend indexer station 210, the substrates are automatically removed fromthe cassette 280 and placed, one at a time, into the outside brushstation 220.

In the outside brush station 220, a substrate is processed through afirst scrub. During the first scrub, the cleaning solution may beapplied to the substrate in several different ways. For example, in oneembodiment the cleaning solution is sprayed onto the substrate. Inanother embodiment the cleaning solution is applied to the substratethrough brushes 221. Yet another embodiment applies the cleaningsolution by dripping the cleaning solution onto the substrate.

The scrubbed substrate is then automatically removed from the outsidebrush station 220 and placed into the inside brush station 230. In theinside brush station 230, the substrate is processed through a secondscrub. In the inside brush station 230 the cleaning solution may beapplied to the substrate in a similar manner as in outside brush station220.

After the second scrub the substrate is then automatically removed fromthe inside brush station 230 and placed into the rinse, spin and drystation 240. Rinse, spin, and dry station 240 rinses, spins, and driesthe substrate. At this point the wafer has been cleaned.

Once the rinse, spin, and dry steps have been completed the substrate isthen transported from the rinse, spin, and dry station 240 to the outputstation 250 where the substrate will be placed into cassette 281. Thetransfer is usually carried out by a robotic arm which lifts thesubstrate out of the rinse, spin, and dry station 240 by its edges andplaces it into the cassette 281. The cassette is then transferred tostorage or to another cleaning or processing system.

It will be clear to one of ordinary skill in the art that some of thesteps in the cleaning system described above may occur in another orderand/or with various solutions depending upon the substrate or substratelayer being cleaned. For example, different cleaning solutions, such aswater, citric acid, ammonium hydroxide, ammonium citrate, andhydrofluoric acid solution (or mixtures of solutions) may be used in oneof the brush stations. Also, other systems may include one brushstation, or more than two brush stations. Moreover, other systems mayomit one or more of the above stations/steps and may include additionalprocessing stations, such as a CMF station.

It should be noted that while the following description illustrates theuse of the present invention in a cleaning system in which both sides ofthe substrate are scrubbed simultaneously, the present invention may beused in other cleaning systems and processes. For example, a cleaningsystem in which only a single side of the substrate is scrubbed or acleaning system in which the substrate is cleaned with chemical spray.

FIG. 3 illustrates one embodiment of the process of the presentinvention. At step 310, the copper layer is planarized using chemicalmechanical polishing. It should be noted that other techniques forplanarization of the copper layer may be used and that it may still bedesirable to clean the semiconductor substrate using the presentinvention after such planarization in order to remove potentialcontaminants from the substrate surface and/or subsurface.

At step 320, the polished semiconductor substrate is then placed in ascrubber. The substrate is then scrubbed, at step 330, to remove thecontaminants caused by the polishing process. During scrubbing, acleaning solution is applied to the substrate in order to aid and/oreffectuate the removal of the contaminants (step 340). This cleaningsolution may be used in either outside brush station 220 or inside brushstation 230, or both brush stations if necessary, of the scrubber inFIG. 2.

In one embodiment, the present invention uses a cleaning solution thatis made up of deionized water, an organic compound, and a fluoridecompound, all of which are combined in an acidic pH environment forcleaning the surface of a semiconductor substrate after polishing acopper layer. The use of an acidic pH environment helps dissolve copperoxide and alleviates some of the problems of brush loading discussed inthe background of the invention. It should be noted that it is desirableto keep the acidic pH environment within a pH level range ofapproximately 1-6. In one embodiment of the present invention, theacidic pH environment has a pH level in the range of approximately 2-4.

The use of an organic compound helps to change the electrostatic forcesbetween the particles and surfaces of the brush and substrate in orderto make them repulsive. Thus, the particles repel the brushes and thesubstrate and the substrate and brushes repel the particles providingfavorable conditions for particle removal. The organic compound used maybe an organic acid, the ammonium salt of an organic acid, or an anionicsurfactant. Some examples of potential organic acids may be: citricacid, malic acid, malonic acid, succinic acid, or any combination ofsuch organic acids.

It should be noted that it is desirable to dissolve the organic compoundin deionized water (DIW) in a concentration range of approximately 100ppm to 2% by weight. In one embodiment of the present invention theorganic compound is dissolved in DIW such that the concentration rangeis approximately 200 ppm to 0.2% by weight.

The use of a fluoride compound helps to remove the contaminants from thesurface and subsurface of the dielectric layer. The present inventionincorporates the use of fluoride compounds such as hydrogen fluorideacid (HF) or ammonium fluoride (NH₄ F) in the cleaning solution. HF orbuffered HF (ammonium fluoride mixed with HF) etches silicon dioxide.Thus, during the scrubbing process the silicon dioxide layer is alsoetched. For example during the brush scrubbing process, typicallybetween 10-100 Å of the oxide layer is removed, leaving clean,uncontaminated dielectric surface on the substrate.

It should be noted that it is desirable to dissolve the fluoridecompound in deionized water (DIW) in a concentration range ofapproximately 0.1% to 5% by weight. In one embodiment of the presentinvention the fluoride compound is dissolved in DIW such that theconcentration range is approximately 0.2% to 1% by weight.

The cleaning solution of the present invention is a mixture of chemicalsin DIW containing a fluoride compound such as HF or NH₄ F; an organicacid, an ammonium salt of an organic acid, or an anionic surfactant; inan acidic pH environment. It should be noted, however, that hydrochloricacid (HCl) may also be added to the solution to adjust pH and helpdissolve copper oxide. It should also be noted that the chemicals of thepresent invention are pre-mixed in the same cleaning solution tosimultaneously solve several problems related to post copper CMPcleaning using a brush scrubber. Cross contamination from substrate tosubstrate and within the same substrate are therefore reducedsubstantially, or even prevented in this simple approach.

One example of the many different ways to formulate the cleaningsolution is: 0.5% HF, 0.1% Citric Acid, and 0.4% NH₄ OH weight mixed inDIW. The pH level of the solution in this example is approximately 3. Itshould be noted however, that the present invention covers variousformulations of the cleaning solution, and that each component in thesolution may be replaced by different chemical that has similarproperties.

Thus, methods and apparatus for cleaning semiconductor substrates afterpolishing of copper film have been described. Although specificembodiments, including specific equipment, parameters, methods, andmaterials have been described, various modifications to the disclosedembodiments will be apparent to one of ordinary skill in the art uponreading this disclosure. Therefore, it is to be understood that suchembodiments are merely illustrative of and not restrictive on the broadinvention and that this invention is not limited to the specificembodiments shown and described.

What is claimed is:
 1. A method to remove contaminants from asemiconductor substrate, comprising:placing the semiconductor substratehaving a polished copper layer in a scrubbing apparatus; and scrubbingthe semiconductor substrate in an acidic cleaning solution formed bymixing about 100 ppm to about 2% by weight of an organic acid selectedfrom the group consisting of citric acid, malic acid, malonic acid,succinic acid, and mixtures thereof and about 0.1% by weight to about 5%by weight of a fluoride compound selected from the group consisting ofhydrogen fluoride, ammonium flouride, and buffered hydrogen fluoride(ammonium fluoride mixed with hydrogen fluoride) in deionized water, theacidic cleaning solution having a pH in a range from about 2 to about 4.2. The method of claim 1, wherein the acidic cleaning solution is abuffered acidic cleaning solution.
 3. The method of claim 1, wherein theacidic cleaning solution is formed by mixing about 200 ppm to about 0.2%by weight of the organic acid and about 0.2% by weight to about 1% byweight of the fluoride compound in deionized water.
 4. The method ofclaim 2, wherein the organic acid is citric acid and the fluoridecompound is buffered hydrogen fluoride (ammonium fluoride mixed withhydrogen fluoride).
 5. The method of claim 1, wherein the acidiccleaning solution is formed by mixing about 0.1% by weight of citricacid and about 0.2% by weight to about 1% by weight of buffered hydrogenfluoride (ammonium fluoride mixed with hydrogen fluoride) in deionizedwater.
 6. The method of claim 5, wherein the buffered hydrogen fluorideis comprised of about 0.1% by weight to about 0.5% by weight of ammoniumfluoride and about 0.1% by weight to about 0.5% by weight of hydrogenfluoride.
 7. A scrubber for processing a semiconductor substrate,comprising:an input to receive a semiconductor substrate having apolished copper layer; a brush assembly coupled to the input; and acleaning solution delivery system for delivering a cleaning solutionformed by mixing about 100 ppm to about 2% by weight of an organic acidselected from the group consisting of citric acid, malic acid, malonicacid, succinic acid and mixtures thereof and about 0.1% by weight toabout 5% by weight of a fluoride compound selected from the groupconsisting of hydrogen fluoride, ammonium flouride, and bufferedhydrogen fluoride (ammonium fluoride mixed with hydrogen fluoride) indeionized water, the cleaning solution having a pH in a range from about2 to about 4, wherein the cleaning solution delivery system delivers thecleaning solution premixed in an acidic pH environment to thesemiconductor substrate having the polished copper layer.
 8. Thescrubber of claim 7, wherein the cleaning solution is formed by mixingabout 200 ppm to about 0.2% by weight of the organic acid and about 0.2%by weight to about 1% by weight of the fluoride compound in deionizedwater.
 9. The scrubber of claim 8, wherein the organic acid is citricacid and the fluoride compound is buffered hydrogen fluoride (ammoniumfluoride mixed with hydrogen fluoride).
 10. The scrubber of claim 7,wherein the cleaning solution is formed by mixing about 0.1% by weightof citric acid and about 0.2% by weight to about 1% by weight ofbuffered hydrogen fluoride (ammonium fluoride mixed with hydrogenfluoride) in deionized water.
 11. The scrubber of claim 10, wherein thebuffered hydrogen fluoride is comprised of about 0.1% by weight to about0.5% by weight of ammonium fluoride and about 0.1% by weight to about0.5% by weight of hydrogen fluoride.
 12. A method to remove contaminantsfrom a semiconductor substrate, comprising:placing the semiconductorsubstrate having a polished copper layer in a scrubbing apparatus; andscrubbing the semiconductor substrate in a cleaning solution formed bymixing about 0.1% by weight of citric acid and about 0.2% by weight toabout 1% by weight of buffered hydrogen fluoride (ammonium fluoridemixed with hydrogen fluoride) in deionized water, the cleaning solutionhaving a pH of about
 4. 13. The method of claim 12, wherein the bufferedhydrogen fluoride is comprised of about 0.1% by weight to about 0.5% byweight of ammonium flouride and about 0.1% by weight to about 0.5% byweight of hydrogen fluoride.
 14. A scrubber for processing asemiconductor substrate, comprising:an input to receive a semiconductorsubstrate having a polished copper layer; a brush assembly coupled tothe input; and a cleaning solution delivery system for delivering acleaning solution formed by mixing about 0.1% by weight of citric acidand about 0.2% by weight to about 1% by weight of buffered hydrogenfluoride (ammonium fluoride mixed with hydrogen fluoride) in deionizedwater, the cleaning solution having a pH of about 4, wherein thecleaning solution delivery system delivers the cleaning solutionpremixed in an acidic pH environment to the semiconductor substratehaving the polished copper layer.
 15. The scrubber of claim 14, whereinthe buffered hydrogen fluoride is comprised of about 0.1% by weight toabout 0.5% by weight of ammonium flouride and about 0.1% by weight toabout 0.5% by weight of hydrogen fluoride.